Minimal implementation in Python of C.J.Date's ELLIPSE - CIRCLE type hierarchy. TYPE ELLIPSE POSSREP (A LENGTH, B LENGTH, CTR POINT) CONSTRAINT A >= B); TYPE CIRCLE IS ELLIPSE CONSTRAINT THE_A(ELLLIPSE) = THE_B(ELLIPSE) POSSREP (R = THE_A(ELLIPSE), CTR = THE_CTR(ELLIPSE));

ellipse_circle.py

#!/usr/bin/python


class Point(object):
    def __init__(self, x, y):
        assert isinstance(x, int)
        assert isinstance(y, int)
        self.x = x
        self.y = y

    def __repr__(self):
        return "Point(x=%s, y=%s)" % (self.x, self.y)


class Ellipse(object):
    def __init__(self, a, b, ctr):
        assert isinstance(a, int)
        assert isinstance(b, int)
        assert isinstance(ctr, Point)
        assert a >= b
        self.a = a
        self.b = b
        self.ctr = ctr

    def __repr__(self):
        return "Ellipse(a=%s, b=%s, ctr=%r)" % (self.a, self.b, self.ctr)

    def __setattr__(self, name, value):
        if not hasattr(self, 'ctr'):
            self.__dict__[name] = value
            return
        if name == 'a':
            assert value >= self.b
        elif name == 'b':
            assert self.a >= value
        self.__dict__[name] = value


class Circle(Ellipse):
    def __init__(self, r, ctr):
        assert isinstance(r, int)
        assert isinstance(ctr, Point)
        super(Circle, self).__init__(r, r, ctr)
        self.r = r

    def __setattr__(self, name, value):
        if hasattr(self, 'r') and (name == 'a' or name == 'b'):
            raise KeyError
        elif name == 'r':
            super(Circle, self).__setattr__('a', value)
            super(Circle, self).__setattr__('b', value)
        else:
            super(Circle, self).__setattr__(name, value)

    def __repr__(self):
        return "Circle(r=%s, ctr=%r)" % (self.a, self.ctr)

if __name__ == '__main__':
    p = Point(0, 0)
    e = Ellipse(4, 2, p)
    c = Circle(1, p)
    print("Initial")
    print(p)
    print(e)
    print(c)
    print()
    e = c
    print("After e = c")
    print(e)
    print(c)
    print()
    e.a = 3
    print("After e.a = 3")
    print(e)
    print(c)